The Summon Stone is a passive item found in Momodora III.
Equipping the Summon Stone will create a doppelganger ghost of your character. The ghost will follow you around, if no enemy is in sight and in case of being to far away from you, it will just teleport right behind you.
Upon seeing an enemy, the ghost will react very aggresively and will try to reach and kill the enemy. It will go as far as jumping into pits, if it's actually able to hit an enemy on the way there. It can't do charge attacks and will always attack with the maple leaf.
While being indestructible, the ghost can still take damage. Upon reaching enough damage to kill it, it will instead pause for a short while before awakening with full health again.
The Summon Stone is a very useful item, since it can take care of a lot of enemies on it's own. There are certain scenarios where the ghost has a hard time to reach the enemy like the second phase of Rell's boss fight, but it normally will be worth it.